1. Field of the Invention
The present invention relates to a two-output inverter transformer having a primary winding located in the middle and secondary windings located on both sides thereof, and more specifically to an inverter transformer having variation in inductance between the secondary windings reduced by devices applied to the sizes and positional relationship of a pole-like core and a rectangle frame-like core forming a magnetic path. This inverter transformer is useful for the back light of a liquid crystal display apparatus, for example.
2. Related Art
For a back light for liquid crystal television sets or liquid crystal displays, a plurality of cold cathode tubes are used. For example, 32-inch liquid crystal television sets have 16 cold cathode tubes arranged at appropriate intervals on their back, thus keeping the brightness of their entire screen. When a plurality of cold cathode tubes are turned on, in order to suppress variation in brightness between the cold cathode tubes to achieve even illumination, the lamp currents of the individual cold cathode tubes need to be kept uniform. If each cold cathode tube is driven by a transformer independently, variation in brightness will be easily suppressed. However, because the drive circuit becomes complex, it is inefficient. Accordingly, it has been proposed that two cold cathode tubes are configured to be driven by one transformer, thereby reducing the number of the components and the size of the entire inverter circuit thus lowering costs.
A transformer for back-light having two outputs on the secondary side of its high-voltage transformer is described in, for example, Japanese Utility Model Application Publication No. H07-22528. Usually, an inverter transformer and a drive circuit are integrated into a unit, and the placement of the unit as a cold cathode tube turning-on circuit is repeated on the back and sides of a set of cold cathode tubes a number of times according to the size of a liquid crystal panel (or the number of cold cathode tubes arranged), and the units are wired to the cold cathode tubes as needed, thereby producing a liquid crystal panel.
As to liquid crystal television sets or liquid crystal displays, the brightness of the screen is becoming increasingly high, and correspondingly variation in brightness between the plurality of cold cathode tubes is becoming a bigger problem. In particular, when a two-output inverter transformer is used for a reduced size and lowered cost, because inductance variation between the two secondary windings (output windings) would cause variation in lamp current, the inductance variation needs to be suppressed.
Accordingly, in Japanese Utility Model Application Publication No. H07-22528, a bobbin, cores, a gap sheet, and the like are configured to be mirror-symmetrical with respect to the center, thereby reducing inductance variation between the two secondary windings. However, there are the problem in assembly that two C-shaped cores and an I-shaped core are put together to touch each other and fixed and the problem that the size tolerance of the C-shaped cores and I-shaped core causes a difference in gap size and thus the inductance variation between the two secondary windings cannot be reduced enough.
Meanwhile, a technique has been proposed in which a magnetic path is formed by a pole-like core and a rectangle frame-like core, but while the gap size can be strictly controlled with use of a gap sheet, the inductance variation between the two secondary windings cannot be reduced enough.
Furthermore, because one liquid crystal panel uses a plurality of inverter transformers, in order to reduce the unevenness of brightness, variation in inductance between the secondary windings of the inverter transformers needs to be suppressed when mass-produced. However, in an actual mass-production process, because of the accuracy of the core sizes, positional deviations of the cores when combined, variation in the state of winding, and the like, variation in inductance is inevitable.